Eddy variability and energetics from direct current measurements in the Antarctic Circumpolar Current south of Australia

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Phillips, HE and Rintoul, SR
(2000)
Eddy variability and energetics from direct current measurements in the Antarctic Circumpolar Current south of Australia.
Journal of Physical Oceanography, 30 (12).
pp. 3050-3076.
ISSN 1520-0485

Abstract

Two year time series measurements of current velocity and temperature in the Subantarctic
Front (SAF) south of Australia from 1993 to 1995 provide estimates of eddy fluxes of heat
and momentum across the Antarctic Circumpolar Current (ACC) and further insight into the
variability of the ACC. The SAF was found to be an energetic, meandering jet with vertically
coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm/s
at 1150 dbar. A daily-varying coordinate frame which follows the direction of ow allowed
mesoscale variability of the SAF to be isolated from variability due to meandering of the
front and proved very successful for examining eddy uxes. Vertically averaged cross-stream
eddy heat flux was 11.3 kW/m^2 poleward and significantly different from zero at the 95%
confidence interval for fluctuations in the 2-90 day band. Zonally integrated, this eddy heat
flux (=0.9 PW) is more than large enough to balance heat lost south of the Polar Front
and is as large as cross-SAF fluxes found in Drake Passage. Cross-stream eddy momentum
fluxes were small, not significantly different from zero and of indeterminate sign, but tending
to decelerate the mean flow. A relationship between vertical motion and meander phase
identified in the Gulf Stream was found to hold for the SAF. Eddy kinetic energy levels were
similar to those in Drake Passage and southeast of New Zealand. Eddy potential energy was
up to an order of magnitude larger than other ACC sites, most likely because meandering
of the front is more common here. Baroclinic conversion was found to be the dominant
mechanism by which eddies grow south of Australia. Typical eddy growth rate is estimated
to be 30 days, approximately twice as fast as in Drake Passage, consistent with eddy energy
growing rapidly downstream.